The invention relates to a trench transistor structure.
The development of DMOS (double diffused metal oxide semiconductor) or MOS (metal oxide semiconductor) field effect transistors is fundamentally driven by the minimization of the surface area specific resistance Ron·A, since this allows an increase in the static power loss in the turned-on state and hence in the maximum continuous current-carrying capability.
In the case of a DMOS trench transistor structure, trenches spaced by mesa regions define a cell array of trench transistors. The surface area of a transistor cell is therefore made up of a portion of the mesa region and a portion of the trench. In the case of the trench transistors, the mesa region usually contains a source region embedded in a body region of the opposite conductivity type, the body region being formed above a drain region and drift region of the trench transistor. The conductivity in a channel region adjoining the trench in the body region is controlled using a gate electrode which is in a form insulated from the mesa region in the trench.
One possibility for reducing the surface area specific turn-on resistance Ron·A is provided by reducing the dimensions of a transistor cell, i.e. the spacing (also called pitch) between adjacent transistor cells. The pitch can be reduced by narrowing the trench and/or alternatively the mesa region. In the case of DMOS trench transistor structures with a field electrode arrangement, however, the width of the trench is determined essentially by the thickness of a field oxide in the trench, said field oxide insulating the field electrode from the drift zone. The thickness of the field oxide is defined essentially by the voltage drop which is to be expected across it, and becomes greater as the voltage drop increases.
DE 10339455 discloses the practice of connecting one or more field electrodes to such fixed potentials that the voltage drop to be expected across the field oxide is reduced and said field oxide can therefore be in a thinner form without having to accept losses in the withstand voltage of the transistor structure and the service life of the field oxide.
Further patent applications submitted to the German Patent and Trademark Office by the applicant on the date of this application propose allowing the field plates in the trench to float in order thus to achieve the advantages of a thinner field oxide which have been described above.
However, floating field electrodes entail problems with regard to long term stability. Inhomogeneities in the insulation structure between adjacent field electrodes mean that leakage currents can flow between the field electrodes, particularly at high temperatures, as a result of which individual field electrodes can adopt undefined potentials. This results in the loss of blocking capability in the trench transistor structure and hence in failure of the application.
For these and other reasons, there is a need for the present invention.